Ka and Kb Knowledge-Based Practice

Questions

1. 1.

   Which expression correctly represents the base dissociation constant, $K_b$, for methylamine ($\text{CH}_3\text{NH}_2$) in an aqueous solution?

   • A) $K_b = \frac{[\text{CH}_3\text{NH}_2][\text{OH}^-]}{[\text{CH}_3\text{NH}_3^+]}$
   • B) $K_b = \frac{[\text{CH}_3\text{NH}_3^+][\text{H}_3\text{O}^+]}{[\text{CH}_3\text{NH}_2]}$
   • C) $K_b = \frac{[\text{CH}_3\text{NH}_3^+][\text{OH}^-]}{[\text{CH}_3\text{NH}_2][\text{H}_2\text{O}]}$
   • D) $K_b = \frac{[\text{CH}_3\text{NH}_3^+][\text{OH}^-]}{[\text{CH}_3\text{NH}_2]}$
2. 2.

   In an aqueous system, the acid dissociation of $\text{NH}_4^+$ has an equilibrium constant $K_a$, and the base dissociation of its conjugate base $\text{NH}_3$ has an equilibrium constant $K_b$. Which chemical equation represents the net equilibrium process whose equilibrium constant is equal to the product $K_a \times K_b$?

   • A) $\text{NH}_4^+(\text{aq}) + \text{OH}^-(\text{aq}) \rightleftharpoons \text{NH}_3(\text{aq}) + \text{H}_2\text{O}(\text{l})$
   • B) $\text{NH}_3(\text{aq}) + \text{H}_3\text{O}^+(\text{aq}) \rightleftharpoons \text{NH}_4^+(\text{aq}) + \text{H}_2\text{O}(\text{l})$
   • C) $2\text{H}_2\text{O}(\text{l}) \rightleftharpoons \text{H}_3\text{O}^+(\text{aq}) + \text{OH}^-(\text{aq})$
   • D) $\text{NH}_4^+(\text{aq}) + \text{H}_2\text{O}(\text{l}) \rightleftharpoons \text{NH}_3(\text{aq}) + \text{H}_3\text{O}^+(\text{aq})$
3. 3.

   For a diprotic acid $\text{H}_2\text{A}$ with acid dissociation constants $K_{a1}$ and $K_{a2}$, and its conjugate base $\text{A}^{2-}$ with successive base dissociation constants $K_{b1}$ and $K_{b2}$, which expression correctly equals $K_w$ at 25 °C?

   • A) $K_w = K_{a1} \times K_{b1}$
   • B) $K_w = K_{a1} \times K_{b2}$
   • C) $K_w = K_{a2} \times K_{b2}$
   • D) $K_w = K_{a1} \times K_{a2}$
4. 4.

   Four weak monoprotic acids have the following acid dissociation constants at 25 °C: $\text{HA}$ ($K_a = 7.2 \times 10^{-4}$), $\text{HB}$ ($K_a = 1.8 \times 10^{-5}$), $\text{HC}$ ($K_a = 4.0 \times 10^{-7}$), and $\text{HD}$ ($K_a = 6.3 \times 10^{-10}$). Which of their conjugate bases has the largest base dissociation constant ($K_b$) at this temperature?

   • A) $\text{A}^-$
   • B) $\text{D}^-$
   • C) $\text{B}^-$
   • D) $\text{C}^-$
5. 5.

   At 25 °C, a monoprotic weak acid $\text{HA}$ has an acid dissociation constant of $K_a = 2.5 \times 10^{-5}$. What is the base dissociation constant, $K_b$, of its conjugate base $\text{A}^-$?

   • A) $2.5 \times 10^{-9}$
   • B) $4.0 \times 10^{-5}$
   • C) $2.5 \times 10^{-10}$
   • D) $4.0 \times 10^{-10}$

Answers and step-by-step solutions appear after you finish the test.